There are many applications which require integrated circuit with configurable interconnect networks. One such application is a FPGA (Field Programmable Gate Array) or field programmable logic cell array where logic cells are to be connected to each other by configurable interconnect networks. Functioning either as a stand-alone chip or as a core part in a system, configurable logic cell arrays are widely used in numerous microelectronic devices.
A conventional interconnect network for field programmable (FP) logic cell array is in planar structure, in which connection cells are configured to connect logic cells of the array to switch cells and connections are made between switch cells and their respective neighboring switch cells. Such a planar interconnect network is quite easy to build physically, but not straightforward to implement logic functions. The interconnect network would become quite complex in order to connect an arbitrary logic cell to another cell of the array. Also, the planar interconnect, network may result in a long routing delay for worst case, which is proportional to the square root of N, wherein N is number of cells in the logic cell array. As the array becomes larger, timing would become more critical and problematic. In addition, the planar network lacks scalability. As the network expands, every switch cell should be expanded to accommodate changing interconnection demands.
Logically, tree-based network can provide a better solution for interconnection of logic cells. Such a tree-based interconnect network, is illustrated in FIG. 1. In FIG. 1, logic cells act as leaves of the tree. Neighboring leaves are connected to first-level switch boxes, and neighboring switch boxes of lower levels are connected to higher-level switch boxes. However, it is difficult to map such a tree-based interconnect network to a highly integrated circuit, which is generally square-shaped.
U.S. Pat. Nos. 6,693,456 and 6,940,308, disclosed a tree-based hierarchical interconnection architecture for integrated circuits, in which logic cells are in row-column layout and interconnections therebetween are of tree structure. The hierarchical interconnect architecture brought forth high efficiency and scalability in interconnection network.
A problem arises when trying to connect logic cells in FP logic cell array to external logics in such a tree-based interconnect network. That is, only peripheral logic cells could be connected to external logics by using conventional method due to limited metal routing tracks. For an L*L FP logic cell array, where L is number of logic cells per row or column, only 4L-4 logic cell could interact with external logics. As FP array and interconnection thereof become larger, utilizing efficiency 4L/L2=4/L decreases dramatically.